If you aren’t familiar with the term inciting incident yet, it can be defined as the beginning of your plot. This doesn't mean the beginning of your book. The book can begin in a lot of different places. But the inciting incident is where you make the promises to your readers about what is going to happen in the climax. It is the moment that sets off your plot.
This moment is often something that happens to the main character, not a decision they make themselves. The evil overlord kidnaps a loved one, the love interest enters the coffee shop, the crazy boss decides to hire the main character, a letter arrives at the Dursleys'. The important thing is that this moment sets up a problem or a question that the main character needs to deal with for the rest of the book.
Inciting incidents can be hard to identify. As I was writing this post I was trying to think of some examples from well-known literature and realized that I don't usually consciously pay attention to the inciting incident. At some point I've just figured out what the story is about and that's good enough for me.
But that's never the case when I'm editing a book. My brain spends the first few chapters desperately searching for an inciting incident to latch onto. Because once I've found that, it is much easier to give helpful feedback about which characters and chapters are working towards the promised climax.
The inciting incident can happen in the first sentence or it can happen in chapter five. It depends on the story you are telling and how much preparation the readers need to really understand the main character's goals and motivations.
For example, in Pride and Prejudice, the inciting incident is when Mr. Bingley falls for Jane at the dance. This is the moment when Elizabeth's world changes. But before readers can fully understand why this effects Elizabeth and what is at stake, there are a few things they need to know.
Mrs. Bennett is a crazy matchmaker
Jane is sweet and shy and mostly does what her mother wants
Marrying for any reason other than love is a terrible fate (as portrayed by the dramatic, humorous, but obviously miserable relationship between Mr. and Mrs. Bennett)
Mr. Bingley is a catch (as portrayed by Mrs. Bennett's immediate assumption that he will marry one of her daughters)
A good inciting incident needs to have personal stakes for the main character. That is the reason that they set off on their quest, whatever that may be.
If you are having trouble identifying the inciting incident of your story, consider the state of your world before your story began. Was the evil overlord already in power? Did the couple know each other? Had the aliens arrived? Main characters don’t just wake up one morning and think, “Wow, I’d really like to take down Darth Vader today.” Something has to happen to take them from their normal world and send them on an adventure.
One plot outlining processes I’ve come across describes moments like these as doors. Once the character steps through, they no longer have access to the world that existed for them before. Readers know that the character won’t be satisfied until this problem is resolved one way or another. Whatever your story is about, that moment of change, that moment where the character gets pointed in a new direction, that is your inciting incident.
If you don’t have one, that’s a problem.
Think of it this way, if Darcy and Bingley never showed up in Pride and Prejudice, the story would be about Elizabeth’s life as usual: her sisters are dramatic, her mother is a little bit nuts, and her father is witty and easily amused. You would watch the characters go to parties and make the same nuisance of themselves they made last week. Elizabeth would still believe she would never fall in love, she’d still have Jane as her eternal confidante, she’d still have Charlotte to complain to (at least until Mr. Collins showed up, which would then become the inciting incident). Pride and Prejudice without the arrival of these three new characters—four if we count Mr. Wickham as well—would be a boring book. Sure, there would be conflict. But it would be the same conflict as last week and we’d have no sense that anything was going to be different in chapter ten. We’d leave the book feeling like Mr. Bennet does after spending too long listening to his wife. “Everything is the same as it was yesterday. What is all the fuss about?”
The inciting incident is a promise to readers that things are changing, that they are going somewhere. When the inciting incident is done correctly, it not only brings the characters into the story, but it brings the readers in as well.